Letter pubs.acs.org/OrgLett
Regio- and Stereoselective Hydrosilylation of Unsymmetrical Alkynes Catalyzed by a Well-Defined, Low-Valent Cobalt Catalyst Alejandro Rivera-Hernández,‡ Brendan J. Fallon,‡ Sandrine Ventre, Cédric Simon, Marie-Hélène Tremblay, Geoffrey Gontard, Etienne Derat, Muriel Amatore, Corinne Aubert, and Marc Petit* UPMC UNIV Paris 06, Institut Parisien de Chimie Moléculaire, UMR CNRS 8232, Case 229, 4 Place Jussieu, Paris 75252 Cedex 05, France S Supporting Information *
ABSTRACT: Herein, the use of a well-defined low-valent cobalt(I) catalyst [HCo(PMe3)4] capable of performing the highly regio- and stereoselective hydrosilylation of internal alkynes is reported. The reaction can be applied to a variety of hydrosilanes, symmetrical and unsymmetrical alkynes, giving in many cases a single hydrosilylation isomer. Experimental and theoretical studies suggest the key step to be a hydrocobaltation and that the reaction proceeds through a classical Chalk−Harrod mechanism.
T
Scheme 1. Cobalt-Catalyzed Hydrosilylation of Alkynes
ransition metal-catalyzed hydrosilylation of alkynes represents one of the most straightforward methods to obtain synthetically valuable vinylsilanes.1 However, a pertinent issue impeding the widespread use of this method is the difficultly in achieving regio- and stereocontrol, both of which are still currently challenging.2 There have been several reports over the past decade on the transition-metal-catalyzed hydrosilylation of alkynes largely based on the use of noble metals (Y,3 Ru,4 Rh,5 Pt).6 In contrast, reports on the use of earth abundant metals (Fe,7 Ni,8 Cu)9 have been sparse, and considering the need to develop new efficient, economic, and environmentally benign catalysts this remains an appealing area of study. Owing to its high abundancy, low cost, and rich redox chemistry, cobalt offers an attractive alternative to other transition-metal-based catalysts. While several groups using low-valent cobalt catalysis recently explored the hydrosilylation of alkenes, reports on alkynes remain rare in comparison.10 The group of Konno demonstrated the catalytic activity of Co2(CO)8 toward the highly regio- and stereoselective hydrosilylation of fluoroalkylated alkynes with HSiEt3 (Scheme 1a).11a Isobe initially reported the hydrosilylation of unsymmetrical alkynes which suffered from poor regioselectivity.11b Later, he described the syn-α-hydrosilylations with internal phenylthioalkynes (Scheme 1b).11c−e Butenschön described the hydrosilylation of a narrow range of internal alkynes using both HSi(OEt)3 and HSiEt3 albeit with poor control of the regioselectivity (Scheme 1c).11f Recently, Deng and co-workers reported the use of low-coordinated cobalt(I) complexes stabilized by a bulky N-heterocyclic carbene ligand (Scheme 1d). Such complexes have proven to be highly efficient for the regio- and stereoselective hydrosilylation of a large variety of alkynes. However, the reaction was only compatible with © XXXX American Chemical Society
H2SiPh2; switching to a tertiary hydrosilane led to a complete shutdown of the catalytic activity.11g Our group has recently been interested in the application of well-defined low-valent cobalt complexes, in particular, HCo(PMe3)4 and Co(PMe3)4, toward C−H bond activation and [2 + 2 + 2] enediyne cycloadditions.12,13 Herein, we report the highly regio- and stereoselective hydrosilylation of internal alkynes. The present study is notable for a number of features: (i) high regio- and stereoselectivities have been obtained for a large variety of unsymmetrical alkynes, (ii) the reaction is compatible with tertiary silanes and alkoxysilanes, and (iii) the one-step synthesis of catalyst is inexpensive and easy on a large Received: July 8, 2016
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DOI: 10.1021/acs.orglett.6b01987 Org. Lett. XXXX, XXX, XXX−XXX
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scale. To begin this study, we first examined the capacity of complex HCo(PMe3)4 to perform the hydrosilylation of diphenylacetylene 1a with various hydrosilanes (Table 1). Table 1. Hydrosilylation of Diphenylacetylene with Various Hydrosilanesa
a
Diphenylacetylene (0.5 mmol), silane (0.55 mmol). bCertain reactions could also be performed at lower temperatures but generally required longer reaction times and led to slightly lower yields (see the Supporting Information). cIsolated yields. dYields obtained using Co(PMe3)4 as catalyst.
Pleasingly, we encountered a reaction amenable with an ample variety of aryl/alkyl tertiary silanes and alkoxysilanes (Table 1, entries 1−7), achieving good yields up to 94%. Notably, these reaction conditions successfully allow for the hydrosilylation of a highly functionalized and sterically demanding silane 2g (Table 1, entry 7). In general the products of syn-hydrosilylation were observed as major isomers, with high (E/Z) ratios. It is worth highlighting the unique ability of this complex to carry out the hydrosilylation of alkynes with a wide variety of hydrosilanes which has not been previously described under cobalt catalysis. As shown in our previous work, terminal alkynes only afford dimerization compounds under these conditions.13a We then turned out attention to the more challenging hydrosilylation of internal unsymmetrical alkynes (Table 2). Such alkynes were substituted by functional groups with different steric hindrances, described as RS (smaller group) and RL (larger group). The complex HCo(PMe3)4 along with the silanes HSi(OEt)3 and HSiPh3 were chosen due to positive preliminary results regarding the high yields and syn selectivity observed. Using less reactive alkynes (compared to diphenylacetylene), new optimized reactions conditions were established, i.e., toluene heated at 160 °C for 2 h (see the SI). Starting the initial scope with the silane 2a (HSiPh3), the product of hydrosilylation of 1-phenyl-1-hexyne was isolated in 92% yield (Table 2, entry 1) in favor of the syn adduct 3ba (product of type 3 with the H atom positioned close to the larger substituent). Variation of the electronics on the phenyl moiety has no effect on the yield, but a better regioselectivity was observed in the presence of an electron-withdrawing group (Table 2, entries 2 and 3). Next, we examined the reactivity of TMS-protected alkynes 1e−k. Previous reports on the hydrosilylation of these kind of alkynes indicated that both (E)-α,β5d,6 or (E)-α,α-disilylalkenes11g products could be selectively obtained. Under our reaction conditions, and using HSiPh3, the hydrosilylation of TMS-protected arylacetylenes 1e−i (Table 2, entries 4−8) led exclusively to the corresponding disilylalkenes in high yields and complete
a
Alkyne (0.5 mmol), HSiR3 (0.55 mmol). bIsolated yields. cFor detailed information regarding the (E/Z) and (α/β) ratios of the obtained products (see the Supporting Information). dReaction was performed at 60 °C for 24 h. eReaction was performed at 100 °C for 24 h. fRegiochemistry of the major compound was not determined.
regioselectivity toward the adducts of type 3, i.e., (E)-α,βdisilylalkenes. The catalytic system also allowed the use of heteroatoms with TMS-protected pyridylalkynes 1j−k, isolating only the products of type 3 in moderate to good yields (Table 2, entries 9 and 10). It is worth noting that the regioselectivity oberved in our reaction with TMS-containing alkynes is opposite to the one previously reported by Deng11g (see below for more detailed discussion). Adding two methyl groups in the orthoposition on a phenyl ring generates a steric bulk on one side of the alkyne, which directs the addition of the H atom to the sterically hindered carbon to give product 3la (Table 2, entry 11). 1-Phenylpropyne 1m followed the same trend of steric dependence of the alkyne substituents in the formation of the final product 3ma. The low yield is presumably due to a stability issue of the alkyne (Table 2, entry 12). On the other hand, the reaction of nonsterically differentiated unsymmetrical alkynes but bearing electronically distinct substituents (1n, 1o, and 1p) gave 50/50 mixtures of vinylsilanes of type 3 and 3′ (Table 2, entries 13−15). These latter results imply that the regio- and stereocontrol of the reaction is predominantly governed by steric factors of the starting substrates, while electronic factors have no significant effect on the outcome of B
DOI: 10.1021/acs.orglett.6b01987 Org. Lett. XXXX, XXX, XXX−XXX
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smallest activation barrier, shown in Figure S1 (see the SI). The main question addressed is whether the alkyne insertion initially occurs into the Co−H bond (hydrocobaltation) or into the Co−Si bond (silacobaltation). According to the results, the barrier for the hydro-obaltation process is 3.18 kcal/mol, while the silacobaltation barrier is 19.42 kcal/mol. Thus, our cobalt catalyst would follow the classical Chalk−Harrod mechanism.18 Another important question addressed by the DFT calculations was that, despite the use of a hydridocobalt catalyst with a silane, no H2 evolution was experimentally observed. This could be explained since starting from the cobalt complex 6 bearing two hydrogen atoms the reductive elimination of H2 is an endothermic process with a transition state residing above by 3.36 kcal/mol, while the reverse barrier (i.e., H2 addition on the cobalt center) is very small (0.47 kcal/mol). Thus, even if H2 is produced during the catalytic process, it will quickly rebound and split onto the cobalt center to regenerate the active species (Figure S2, SI). Based on the experimental and theoretical information (see also Tables S2 and S3 in the SI), we propose the following catalytic cycle (Figure 1). Initial oxidative addition of the silane
the reaction. Finally, the use of ester-substituted substrates 1q and 1r in the hydrosilylation with HSiPh3 yields solely the product of type 3′, presumably due to the chelating effect of the ester which appears to overcome the steric control (Table 2, entries 16 and 17).12,14 We obtained analogous results when we addressed the hydrosilylation of unsymmetrical internal alkynes using the silane HSi(OEt)3 2b, which again gave almost exclusively the syn-adducts of type 3 (Table 2, entries 18−27). This time, products 3bb and 3cb showed complete regioselectivity, presumably due to the larger steric bulk provided by the triethoxy moieties in the silane (Table 2, entries 18 and 19). Further exploration into the scope of the current reaction showed identical results by employing TMSprotected arylacetylenes, with complete selectivity toward the corresponding (E)-α,β-disilylalkenes of type 3 and regardless of the aromatic group linked to the alkyne. Again, alkynes 1l and 1n gave similar regioselectivities as previously observed with triphenylsilane (Table 2, entries 26 and 27). Extension of this methodology to an intramolecular version was also possible, which provides an alternative route to benzosilole derivatives via a hydrosilylation/double migration sequence (Scheme 2).15 Scheme 2. Intramolecular Hydrosilylation
Focusing our attention now on the reaction mechanism, we studied the stoichiometric reaction of hydridocobalt complex HCo(PMe3)4 and triphenylsilane (Scheme 3a). Having Scheme 3. Reaction of HCo(PMe3)4 with HSiPh3
Figure 1. Proposed mechanism.
to the cobalt center, followed by coordination of the alkyne, gives the dihydridocobalt(III) intermediate III, which undergoes alkyne insertion into a Co−H bond to furnish the vinyl organometallic IV. At this point, the H atom is directed toward the more bulky substituent in order to place the cobalt moiety in the less hindered plane of the molecule. Direct reductive elimination releases the vinylsilane V as the major product with the observed regioselectivity and regenerates the catalytically active species I. To a much lesser extent, an opposite migratory insertion would yield the adduct isomer VI, which eventually results in the unfavored product VII. A model to compare the regioselectivity between our results and Deng’s11g is shown in Scheme 4. In the model proposed by Deng, the sila-cobaltation step is controlled by the very bulky ligand IAd (1,3-diadamantylimidazol-2-ylidene), and this determines the regioselectivity of the reaction. For unsymmetrical internal alkynes, this control based on a bulky ligand is efficient when the steric differentiation between the two substituents is significant. Otherwise, it leads to low selectivity as shown by Deng using 1-phenylpropyne which yielded a 50/ 50 mixture of regioisomers. In our system, we proposed that a hydrocobaltation step controls the selectivity. In this process
successfully isolated the dihydrocobalt(III) complex 6, we began to explore its catalytic activity.16,17 To this end, the reaction of diphenylacetlyene and triphenylsilane in the presence of 5 mol % of 6 afforded the desired hydrosilylation product 3aa in good yields and the same selectivity as that observed with HCo(PMe3)4 (Scheme 3b). This result suggests a potential participation of the dihydrodocobalt(III) complex to trigger the catalytic process. Moreover, the stoichiometric reaction of diphenylacetylene with HCo(PMe3)4 shows only a ligand exchange and no evidence of hydrocobaltation (Scheme 3c). To gain deeper insight into the mechanistic details, DFT (density functional theory) calculations were performed on a system based on complex 6 in conjunction with 1-phenylpropyne and HSi(OMe)3. Following our previous studies,13 parameters were set as functional B3LYP and basis set SVP, while all the DFT calculations were performed with Gaussian09. We present the conformation, which led to the C
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(3) (a) Molander, G. A.; Romero, J. A. C.; Corrette, C. P. J. Organomet. Chem. 2002, 647, 225. (b) Schumann, H.; Keitsch, M. R.; Winterfeld, J.; Muhle, S.; Molander, G. A. J. Organomet. Chem. 1998, 559, 181. (4) (a) Fürstner, A.; Radkowski, K. Chem. Commun. 2002, 2182. (b) Adams, R. D.; Barnard, T. S. Organometallics 1998, 17, 2567. (5) (a) Liu, C.; Widenhoefer, R. A. Organometallics 2002, 21, 5666. (b) Hofmann, P.; Meier, C.; Hiller, W.; Heckel, M.; Riede, J.; Schmidt, M. U. J. Organomet. Chem. 1995, 490, 51. (c) Brockmann, M.; tom Dieck, H.; Klaus, J. J. Organomet. Chem. 1986, 301, 209. (d) Sanada, T.; Kato, T.; Mitani, M.; Mori, A. Adv. Synth. Catal. 2006, 348, 51. (6) Chauhan, M.; Hauck, B. J.; Keller, L. P.; Boudjouk, P. J. Organomet. Chem. 2002, 645, 1. (7) (a) Greenhalgh, M. D.; Frank, D. J.; Thomas, S. P. Adv. Synth. Catal. 2014, 356, 584. (b) Greenhalgh, M. D.; Jones, A. S.; Thomas, S. P. ChemCatChem 2015, 7, 190. (8) (a) Bartik, T.; Nagy, G.; Kvintovics, P.; Happ, B. J. Organomet. Chem. 1993, 453, 29. (b) Tillack, A.; Pulst, S.; Baumann, W.; Baudisch, H.; Kortus, K.; Rosenthal, U. J. Organomet. Chem. 1997, 532, 117. (c) Chaulagain, M. R.; Mahandru, G. M.; Montgomery, J. Tetrahedron 2006, 62, 7560. (d) Berding, J.; van Paridon, J. A.; van Rixel, V. H. S.; Bouwman, E. Eur. J. Inorg. Chem. 2011, 2011, 2450. (9) García-Rubia, A.; Romero-Revilla, J. A.; Mauleoń, P.; Arrayaś, R. G.; Carretero, J. C. J. Am. Chem. Soc. 2015, 137, 6857. (10) (a) Brookhart, M.; Grant, B. E. J. Am. Chem. Soc. 1993, 115, 2151. (b) Chen, C.; Hecht, M. B.; Kavara, A.; Brennessel, W. W.; Mercado, B. Q.; Weix, D. J.; Holland, P. L. J. Am. Chem. Soc. 2015, 137, 13244. (c) Sun, J.; Deng, L. ACS Catal. 2016, 6, 290. (d) Schuster, C. H.; Diao, T.; Pappas, I.; Chirik, P. J. ACS Catal. 2016, 6, 2632. (e) Noda, D.; Tahara, A.; Sunada, Y.; Nagashima, H. J. Am. Chem. Soc. 2016, 138, 2480. (f) Ibrahim, A. D.; Entsminger, S. W.; Zhu, L.; Fout, A. R. ACS Catal. 2016, 6, 3589. (11) (a) Konno, T.; Taku, K.; Yamada, S.; Moriyasu, K.; Ishihara, T. Org. Biomol. Chem. 2009, 7, 1167. (b) Hosokawa, S.; Isobe, M. Tetrahedron Lett. 1998, 39, 2609. (c) Huang, K.-H.; Isobe, M. Eur. J. Org. Chem. 2014, 4733. (d) Isobe, M.; Nishizawa, R.; Nishikawa, T.; Yoza, K. Tetrahedron Lett. 1999, 40, 6927. (e) Tojo, S.; Isobe, M. Tetrahedron Lett. 2005, 46, 381. (f) Yong, L.; Kirleis, K.; Butenschön, H. Adv. Synth. Catal. 2006, 348, 833. (g) Mo, Z.; Xiao, J.; Gao, Y.; Deng, L. J. Am. Chem. Soc. 2014, 136, 17414. (12) Ventre, S.; Simon, C.; Rekhroukh, F.; Malacria, M.; Amatore, M.; Aubert, C.; Petit, M. Chem. - Eur. J. 2013, 19, 5830. (13) (a) Ventre, S.; Derat, E.; Amatore, M.; Aubert, C.; Petit, M. Adv. Synth. Catal. 2013, 355, 2584. (b) Fallon, B. J.; Derat, E.; Amatore, M.; Aubert, C.; Chemla, F.; Ferreira, F.; Perez-Luna, A.; Petit, M. J. Am. Chem. Soc. 2015, 137, 2448. (c) Fallon, B. J.; Garsi, J.-B.; Derat, E.; Amatore, M.; Aubert; Petit, M. ACS Catal. 2015, 5, 7493. (d) Fallon, B. J.; Derat, E.; Amatore, M.; Aubert, C.; Chemla, F.; Ferreira, F.; Perez-Luna, A.; Petit, M. Org. Lett. 2016, 18, 2292. (14) The strong electron-withdrawing effect of the ester may also explain the observed selectivity. (15) Onoe, M.; Baba, K.; Kim, Y.; Kita, Y.; Tobisu, M.; Chatani, N. J. Am. Chem. Soc. 2012, 134, 19477. (16) (a) Archer, N. J.; Haszeldine, R. N.; Parish, R. V. J. Chem. Soc. D 1971, 0, 524. (b) Archer, N. J.; Haszeldine, R. N.; Parish, R. V. J. Chem. Soc., Dalton Trans. 1979, 695. (17) For a characterized analogous dihydridocobalt(III) complex, see: Mautz, J.; Heinze, K.; Wadepohl, H.; Huttner, G. Eur. J. Inorg. Chem. 2008, 2008, 1413. (18) Chalk, A. J.; Harrod, J. F. J. Am. Chem. Soc. 1965, 87, 16. (19) Hebden, T. J.; St. John, A. J.; Gusev, D. G.; Kaminsky, W.; Goldberg, K. I.; Heinekey, D. M. Angew. Chem., Int. Ed. 2011, 50, 1873.
Scheme 4. Comparison between RegioselectivityDetermining Steps
the steric differentiation between the silylcobalt moiety and the hydrogen atom is very important. Thus, the regioselectivity is less affected by a change of the silane and by a small steric differentiation between RL and RS (Scheme 4). In summary, a highly regio- and stereoselective hydrosilylation of a wide variety of internal alkynes catalyzed by the inexpensive and versatile low-valent cobalt(I) HCo(PMe3)4 is reported. The analysis of data obtained from NMR and computational studies revealed the direct correlation of steric effects in the outcome of the hydrosilylation products toward a syn-regioselective reaction. Based on our initial investigations, we proposed a mechanism via a hydrocobaltation pathway. Further studies on complex 6 are underway to determine its exact structure as a dihydrogen or a dihydride cobalt complex and to confirm the mechanism.19
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.6b01987. Experimental procedures and analytical data (PDF) Crystallographic data for 3ha, 3ia, and 3la (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions ‡
A.R-H. and B.J.F. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by CNRS, MRES, UPMC, and ANR (ANR-12-BS07-0031- 01COCACOLIGHT), which we gratefully acknowledge. We also thank CONACYT for the postdoctoral fellowship granted to A.R.-H. (Registry No. 252113).
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REFERENCES
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DOI: 10.1021/acs.orglett.6b01987 Org. Lett. XXXX, XXX, XXX−XXX
